212 PROFESSOR HUXLEY ON THE ANATOMY 



advanced that there can be no question as to its real nature. The total length of this mass 

 (which, for reasons which wiU shortly appear more fully, I have termed the generative 

 blastema) is ri^nd of an inch ; and for the greater part of its extent it has the character of 

 indifferent tissue. Bat the sudden enlargement to which I have referred is occupied by a 

 body which has all the characters of an ovum, consisting of a structureless yelk eioth of 

 an inch in diameter, and of a clear germinal vesicle (xaVoth). enclosed in which is a germinal 

 gpot (— gij^-yth). -It will appear by-and-by that this is, in fact, the solitary ovum (sur- 

 rounded by its rudimentary ovisac) which will come to maturity in the bud to be formed 

 at this spot ; and it is not a little remarkable that the first recognizable part of the new 

 organism should be the foundation of that structure which will eventually develope into 

 a creature distinct from it. 



In fig. 15 a more advanced condition of a bud is depicted : the backward continuation 

 of the endostylic cone is broader, more distinctly hollow, and is so bent up as to form a 

 more acute angle, both with the line of direction of the endostyle and with the plane of 

 the generative blastema. In consequence of this change, and of the general enlargement 

 of the parts, they can no longer be contained within the l}lood-sinus, whose outer wall is 

 now elevated into a conical cap which fits over the conjoined ends of the process of the 

 endostyle and the generative blastema. That part of the external timic which consti- 

 tutes this cap is thickened, and exhibits the texture of indifferent tissue. The ovum in 

 the generative blastema is now very distinct, and the tissue around it is so disposed as to 

 mark out the walls of an ovisac which measures i^io^^ oi an inch in diameter. The clear 

 germinal vesicle measures sToth of an inch ; and its spot has the same diameter as before. 

 Behind the ovisac, which occupies the greater part of the cavity of the diverticulum 

 constituting the bud, a distinct constriction marks off the rest of the generative blastema, 

 which lies closely connected mth the external trmic of the parent, and altogether 

 excluded from the cavity of the " cap " of the nascent bud. It is now no longer taper, but 

 cylindrical and rounded at the end ; and near its anterior extremity a new germinal spot, 

 surrounded by a small clear vesicle, is visible. 



Eig. 16 represents a bud giotb. of an inch broad by 4ioth of an inch high. The pro- 

 cess of the endostylic cone is very distinctly hollow and somewhat thin-walled, while its 

 axis is nearly parallel with that of the bud. In fig. 17, the bud, now sub cylindrical, has 

 increased in length to xiTid of an inch ; and the front view of a similar bud, given in 

 fig. 18, shows that the hollow process of the endostylic cone is slightly constricted in the 

 middle, and that the interval between its walls and the external tunic is occupied by a 

 granular mass. 



In fig. 19, a marked advance is discernible. The bud is distinguishable into a body or 

 rudimentary ascidiozooid jT-atb of an inch long, and a much shorter stalk or peduncle. 

 The ascidiozooid is broad at its attached end, more or less tapering at its opposite extremity. 

 Its external tunic is distinct, but proportionably thinner than before, and is continued into 

 the outer wall of the peduncle and thence into the external tunic of the parent. The hol- 

 low process of the endostylic cone is about as broad as before, in the peduncle ; but after 

 traversing this, nearer its anterior than its posterior side, it suddenly dilates into a pyri- 

 form sac, somewhat similar in contour to the rudimentary ascidiozooid itself. The upper 



